The present invention relates to a method of chromatographic chemical analysis, more particularly to the design of multiple fluidic input flow cells for use in photometric measurements.
It is well known to measure an analyte of interest by photometric detection on fluid flowing from one or more chromatographic systems. To effect such measurement at widely varying flow rates presents the need to change flow cells and/or detectors. The changing of flow cells or detectors is both costly and time consuming. Several approaches to solve the problems that occur with varying flow rates with flow cells in the prior art have been attempted with certain limitations.
For example the use of a tee where there are two or more fluidic input lines entering into the tee and a single fluidic line exiting from the tee and into the flow cell is illustrated in FIG. 1. This method can be utilized when the internal diameter of the fluidic line exiting from the tee to the flow cell inlet is sized to handle the flow rates required from the two or more fluidic lines entering the tee. This approach is acceptable if the flow rates of the fluidic lines entering the tee are comparable and the same solvents are being used. A major disadvantage of this method occurs when the flow rates diverge, such as in the case of analytical flow rates which are approximately 1.0 ml/min, and preparative flow rates which are approximately 150 ml/min. If the flow rates are vastly different, such as in the above preparative flow rates, a fluidic line configured for preparative analysis would not be adaptable for analytical use. Attempts to utilize a preparative cell for analytical work results in problems such as chromatographic bandspreading. Conversely, an exiting fluidic line sized for analytical flow rates can cause damage to the column due to the very high backpressures if a preparative use is untertaken. The foregoing problem may be addressed by selecting an internal diameter tubing that will yield acceptable results for both preparative and analytical uses. However, as a practical matter selecting a tubing that will function in both high and low flow rate applications can be difficult to undertake without sacrificing the performance of the flow cell. Additionally, this approach creates the need for multiple fluidic interfaces.
Another method within the prior art is to use two detectors each with its own single inlet flow cell. Each of the flow cells would have different tubing internal diameters sized to accommodate either low or high flow rates. The flow cell for low flow rates could be used for method development and screening while the cell configured for higher flow rates would be used for high throughput analysis and or purification. The problem with this approach is that the detector flow cell combination used for the method development is not the same as that used for analysis or purification. The expense associated with purchasing and maintaining two detectors renders this approach impracticable.
Another approach is to use a single detector and interchange flow cells. In this approach the flow cell utilized is dependent upon the flow rate and type of analysis required. There are several limitations of this approach. The degree of automation within the chromatographic system is reduced because of the need to disturb the detector in order to place a different flow cell within the chromatographic system. Another limitation of this approach is that, while the same detector is used, the potential for variation in the method scale up is high due to utilizing two flow cells.
Consequently, there are numerous limitations associated with prior approaches to solve the problems caused by the variation of flow rates within a chromatography system. Most notably, the prior art suffers from several limitations such as cost, scalability, automation, and validation of the chromatography system.
The present invention provides a chromatographic flow cell having the capacity to handle fluid flow from one or more chromatographic streams, at widely varying flow rates, without the need to change flow cells or detectors.
According to the invention, the flow cell itself acts as a tee where two or more fluidic inlet lines intersect with a common inlet channel. The size of the common inlet channel is such that the diameter and length are configured to minimize the total volume, unswept volume, bandspreading, and backpressure over the varied range of flow rates. The common outlet channel and the outlet line are sized to provide for adequate fluidic handling at the varied flow rates.
The selections of the flow rates used with the flow cell according to the present invention are controlled by the use of directing valves. The user controls the valves either manually or through an automation scheme. The valves are used to direct the sample stream from the injector of the chromatographic system to either a high flow rate or a low flow rate path. Both flow rate paths are in fluidic communication with their respective high or low flow rate inlets of the flow cell. The flow cell contains a common channel that is in communication with both the high and low flow rate inlets. Features of the invention include provisions of a flow cell that does not require a change of detectors over a varied range of flow rates. Bandspreading, for analytical work, is avoided with the flow cell, according to the invention by configuring the common inlet channel with the proper diameter and length. Damage to the column as a result of back pressure is prevented by having a outlet channel that can handle both high and low flow rates. The frequency of validation of the chromatography system is decreased by the utilization of the same flow cell and detector. The cost to the user is greatly reduced by having a single detector that handles both high and low flow rate systems.